For many years, the design of concrete structures imitated the typical steel design of column, girder and beam. With technological advances in structural concrete, however, its own form began to evolve. Concrete has the advantages of lower cost than steel, of not requiring fireproofing, and of its plasticity, a quality that lends itself to free flowing or boldly massive architectural concepts. On the other hand, structural concrete, though quite capable of carrying almost any compressive load, is weak in carrying significant tensile loads. It becomes necessary, therefore, to add steel bars, called reinforcements, to concrete, thus allowing the concrete to carry the compressive forces and the steel to carry the tensile forces.
Structures of reinforced concrete may be constructed with load-bearing walls, but this method does not use the full potentialities of the concrete. The skeleton frame, in which the floors and roofs rest directly on exterior and interior reinforced-concrete columns, has proven to be most economic and popular. Reinforced-concrete framing is seemingly a quite simple form of construction. First, wood or steel forms are constructed in the sizes, positions, and shapes called for by engineering and design requirements. The steel reinforcing is then placed and held in position by wires at its intersections. Devices known as chairs and spacers are used to keep the reinforcing bars apart and raised off the form work. The size and number of the steel bars depends completely upon the imposed loads and the need to transfer these loads evenly throughout the building and down to the foundation. After the reinforcing is set in place, the concrete, a mixture of water, cement, sand, and stone or aggregate, of proportions calculated to produce the required strength, is placed, care being taken to prevent voids or honeycombs.
One of the simplest designs in concrete frames is the beam-and-slab. This system follows ordinary steel design that uses concrete beams that are cast integrally with the floor slabs. The beam-and-slab system is often used in apartment buildings and other structures where the beams are not visually objectionable and can be hidden. The reinforcement is simple and the forms for casting can be utilized over and over for the same shape. The system, therefore, produces an economically viable structure. With the development of flat-slab construction, exposed beams can be eliminated. In this system, reinforcing bars are projected at right angles and in two directions from every column supporting flat slabs spanning twelve or fifteen feet in both directions.
Reinforced concrete reaches its highest potentialities when it is used in pre-stressed or post-tensioned members. Spans as great as one hundred feet can be attained in members as deep as three feet for roof loads. The basic principle is simple. In pre-stressing, reinforcing rods of high tensile strength wires are stretched to a certain determined limit and then high-strength concrete is placed around them. When the concrete has set, it holds the steel in a tight grip, preventing slippage or sagging. Post-tensioning follows the same principle, but the reinforcing tendon, usually a steel cable, is held loosely in place while the concrete is placed around it. The reinforcing tendon is then stretched by hydraulic jacks and securely anchored into place. Pre-stressing is done with individual members in the shop and post-tensioning as part of the structure on the site.
In a typical tendon tensioning anchor assembly used in such post-tensioning operations, there are provided anchors for anchoring the ends of the cables suspended therebetween. In the course of tensioning the cable in a concrete structure, a hydraulic jack or the like is releasably attached to one of the exposed ends of each cable for applying a predetermined amount of tension to the tendon, which extends through the anchor. When the desired amount of tension is applied to the cable, wedges, threaded nuts, or the like, are used to capture the cable at the anchor plate and, as the jack is removed from the tendon, to prevent its relaxation and hold it in its stressed condition.
It is highly desirable to protect the tensioned steel cables from corrosive elements, such as de-icing chemicals, sea water, brackish water, and even rain water which could enter through cracks or pores in the concrete and eventually cause corrosion and loss of tension of the cables. The cables typically are protected against exposure to corrosive elements by surrounding them with a metal duct or, more recently, with a flexible duct made of an impermeable material, such as plastic. The protective duct extends between the anchors and in surrounding relationship to the bundle of tensioning cables. Flexible duct, which typically is provided in 20 to 40 foot sections is sealed at each end to an anchor and between adjacent sections of duct to provide a water-tight channel. Grout then may be pumped into the interior of the duct in surrounding relationship to the cables to provide further protection.
The inventor also has several other patents relating to the introduction of grout into bonded systems or to the placement of ducts around tendons. For example, U.S. Pat. No. 5,720,139, issued on Feb. 24, 1998, describes a method and apparatus for installing a multi-strand anchorage system. The multi-strand anchorage system includes a anchor plate having a front side and a back side, a plurality of tendon-receiving passageways formed in the anchor plate, and a hole formed in the anchor plate and extending so as to open on a front side and a back side of the anchor plate. Each of the tendon-receiving passageways opens on the front side and opens on the back side of the anchor plate. Each of the tendon-receiving passageways tapers so as to having narrow diameter adjacent the back side of the anchor plate and a wide diameter adjacent the front side of the anchor plate. The hole is an unmachined cast hole suitable so that a grout tube can be directly placed therein to deliver grout for the purposes of cementing the multiple tendons within the duct affixed to the anchor plate.
U.S. Design Pat. No. 400,670 issued on Nov. 3, 1998, teaches a particular design of a duct as used in a multi-strand post-tension system. The design is particularly configured so that multiple tendons can be placed therein.
U.S. Pat. No. 5,701,707, issued on Dec. 30, 1997, also a teaches a bonded slab post-tension system including a transition apparatus having a diverter member. The diverter member has a first end and a second end and a tendon port support affixed to the second end of the diverter member. The first end of the diverter member is attached to a duct. The tendon port support has a plurality of tendon ports opening at an end opposite the diverter member. The second end of the diverter member has a greater area than the first end. Each of the tendon ports is of a tubular configuration opening at one end to an interior of the diverter member. A grout connection tube is affixed to a port formed on the top surface of the diverter member so as to allow for the introduction of grout into the duct and around the tendons formed therein.
U.S. Pat. No. 5,775,849, issued on Jul. 7, 1998, teaches a duct system for a post-tension rock anchorage system. This duct system has a first duct with a plurality of corrugations extending outwardly therefrom, a second duct having a plurality of corrugation extending radially outwardly therefrom, and a tubular body threadedly receiving the first duct at one end and threadedly receiving the second duct at the opposite end. The tubular body has a first threaded section formed on an inner wall of the tubular body adjacent one end of the tubular body and a second threaded section formed on an inner wall of the tubular body adjacent an opposite end of the tubular body. The threaded sections are formed of a harder polymeric material than the polymeric material of the first and second ducts. The tubular body has an outer diameter which is less than the diameter of the ducts at the corrugations. The first and second threaded sections have a maximum inner diameter which is less than the outer diameter of the ducts at the end of the ducts. First and second elastomeric seals are affixed to opposite ends of the tubular body and juxtaposed against a surface of a corrugation of the first and second ducts.
It is an object of the present invention to provided a bonded monostrand post-tension system which allows a duct to be easily coupled to an anchor.
It is another object of the present invention to provided a bonded monostrand post-tension system in which grout can be easily introduced so as to fill the spaces within the duct and around the tendon.
It is still a further object of the present invention to provided a bonded monostrand post-tension system which can provide a completely sealed system without voids between the tendon, the duct and the anchor.
It is still a further object of the present invention to provided a bonded monostrand post-tension system which is easy to use, easy to manufacture, and relatively inexpensive.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.